Apparatus for generating coded alternating current



current;

Patented May9, 1944 h 1 UNITED STATES AT OFFICE 2,318,325 l a l I A'PPKRTUS FOR'GENERATINGpdDED ALTERNATING CURRENT San Francisco, Calif.,assignor. to 'l he Uni on Switch and Signalfiompany, S'w'iSsValq'Paacorporation of Pennsylvania Application July 23,1941, SerialNc. 463,631

, solaiz ns. (c1. lit-38b) f'My invention relates to apparaitus forgenerating coded "alternating. 'curr and more particularly to apparatusfor'dernmg- "coded "alterhating current from a source Coded" alternating'cii'rrentis teris'ively insignal system's, for eiia rnple, in signalsystems for railways, When s'uch'currnt is used 'for'si'gn'al systemsfor railwaysa stretch of railway trackis divided; 'intofsuccessi veinsulated tradk'se'tififis, each df whih is DIOVidd with a trackcircuit'to which'coded alternating current.

is supplied for control of cab{signals or wayside signals or both. Thealternating current supplied to each such track circuit is coded atonedifferent traflic conditions. Alternating current of 100'c'ycle's persecond is that commonly used present day signal systems for ir flwaysanothe coding is effected by coders whichperiodically interrupt the circuitat one or another ofdifferent ratesthe code rates 0: 180,- 12c tr-lawsinterruptions per mmutepeirig thosefpleferred; Eachsuch code consists ofalternate: on andpff' periods, current flowing each on "period and "nocurrent flowing each on period Preferably, the on and off periods 'of acode are of substantially equal duration. -Generally speaking the alternating current is 'suppli'edt'o the different track circuits from atransmission line extending along the railway from a central'powerfblanat which bffinidirectional 7 used quite eit or another of difierent 'cod'rats according 'to.

the generator is located; 'I"he"codin'g however is-usually effected by a"coder individual to each track circuit. l h

It has been proposed to generate alternatin current for railway signalsystems by generators located along the railway; one for each controltrack circuit. By such an arrangement the sigrial system is'not subjectto failure due tofst'o'rms and otherweather conditidns that may destroya transmission line. ,Als'o such arrangement avoids the; relatively highcost of a transmission line. When generators, one for each trackcircuit, are usedwthe 'tl'riving energyzis derived from a local batteryzSuch local .generatdrs inust therefore be reliable in:operatiomrelatively low in cost and of rl'ativelyvhigh 'efiiciency inorder to save .on theoutputr-requird"from the battery.

In view oil the circumstances recited above; a

feature .iof. my invention is the provision of novel and,. improvedapparatus for generating coded alternatingscurrerit from. power. derivedfrom a sourcefoflur-iidirectional current;

motl'ierlfeatureof my invention is the provision in apparatus ot thetypehreuinvolved. of

novel, "reliable and inexpensive"means for supplying alternating currentof a preselected irequency' and code rate from power derived from" asource 'of unidirectional current:

" Also afeature of my invention is the provision tus of the type hereinvolved-for effecting "sub-- stantially constant amplitude oscillationsin response to a wave of damped'oscillatiohs;

Again, a feature of my invention is the p'rovision of novel means forderiving a wave or codeimpulse of alternating current of 'apreselectedduration and of substantially uniform amplitudefrom a wave of dampedoscillation.

Other features; advantages and objects of my invention will appear asthe specification progresses.' w v o v i v The above features;advantagesandinoj'ects of my'invention are attained by providinganioscil lating circuit, a magnetically saturablecoupling device, codingmeans and a suitable source of unidirectionalqcurrnt. Thewoscillatingcircuit includes ar'eactor having awinding mounted on a magnetizablecoreandaacrcss which reactor winding a'source of unidirectional current isconnectediover 'a'contact o'f a'code'r having a cyclic operation eachcycle of whichconsists of an on mean ofi'p'eriod. The sourceof currentis connected to the reactor winding each on "code period and isdisconnected therefrom each off code period. Consequently, magneticenergy is stored in the reactor core when unidirectional current'fl'owsin 'the reactor winding each on code period and such stored energydecays while the unidirectional currentisinterrupted ea'ch off codeperiod. The'electrdmotive 'forc'e created in the reactor winding duetothe-decay of the stored energy creates oscillations in the oscillatingcircuit. Such oscillations are of a frequency preselected by theproportion of theparts of the oscillation circuit and are'of a dampedwave, theinitial amplitude of such oscillations beingalsoipredet'ermined by proper proportioning of the parts. Thecouplin'gdevice for coupling this oscillating circuit to the cdntrolcircuit may be a transformer characterized by a saturable magnetic core,a primary winding of the transformer being included in the oscillatingcircuit and a secondary winding ofthei'transformer being connected tothecontrol circuit; The core of:

the transformer saturates when the oscillations flowing in the primarywinding are of a preselected amplitude which is something less than theamplitude of the first few cycles of the oscillations created in theoscillating circuit due to the decay of the magnetic energy stored inthe reactor core. Hence there is induced in the secondary winding of thetransformer oscillations during each off code period of the coder, suchoscillations being of a preselected frequency which is the same as thenatural frequency of the oscillating circuit. Since the transformerisprovided with a, saturable core, the power drawn from the oscillatingcircuit is reduced during the early part of the damped oscillationscreated therein and the rate at which the oscillations die out isreduced. Also the transformer can'be made to feed the required energyfor a short time after the damped oscillations have become too small tosupply the required power. The electro motive force induced in thesecondary winding of the transformer in response to each wave. ofoscillations created in the oscillating circuit has, therefore, anamplitude less than thel'amplitude of the damped oscillations during theearly part of the wave and is maintained at its :initial amplitude for aperiod after the oscillations of the oscillating circuit becomerelatively small. It follows, therefore, that during each. off period ofthe coder, there'issupplied fromthe secondary winding of the transformerto the control circuit, a wave or code impulse of alternating current ofafrequency preselected by the proportion of the oscillating circuit andof substan tially uniform amplitude due to the saturable magneticcharacteristic of the coupling transformer. Furthermore, this codeimpulse of alter-' nating current thus supplied to the control circuitin response to each damped wave of oscil-.

lations created'in the oscillating circuit can be made to persist for aduration substantially equal to theoff code period of the coder. Henceif the on and off code periods of the coder are substantially'eq'ual,the on and off periods of the coded alternating current supplied tothe'con' trol circuit are likewise substantially equal.-

A very sudden dying out of each code impulse of the alternating currentis obtained by having the direct current circuit closed by thecoder atjust the instant'the code impulse of alternating current is to bestopped, because such closing of the direct current circuit shortcircuits the oscillating circuit as far as the flow of the oscillationsis concerned.

A filter tuned sharply at the frequency of the alternating currentderived from the oscillating circuit may be interposed between thetransformer secondary winding and the control circuit to suppressharmonics.

Again, a three legged transformer and a condenser may be usedas thecoupling device. The primary and secondary windings are placed on thetwo outside legs of such core structure and the center leg, with anairgap, serves asa magnetic shunt. The outside leg on which the secondarywinding is mounted is proportioned for magnetic saturation, while thecenter .leg is of larger cross section and doesnot saturate at theamplitude of the oscillations flowing in the primary winding. The firstadvantage of this latter transformer is that when the secondary windingleg is saturated and an increase of the current flowing in the primarywinding causes an increase in the flux, most of this. increment of fluxflows through the un aturated center le and hence the electromotiveforce induced in the secondary winding is retained more constant inamplitude. The second advantage of this threelegged transformer is thatthe leakage reactance due to the magnetic shunt, which is roughlyequivalent to an inductance in series with the secondary winding, tendsto filter the output and reduce harmonics. The condenser associated withsuch transformer is connected across the primary winding and serves thepurpose of tuning out the higher inductance of the primary winding atlow-current, and thus prevents frequency variations in the oscillatingcircuit. Such condenser also maintains the required value of primarywinding current after the energy in the reactor has died'down to a lowvalue and hence aids in maintaining the code impulses of alternatingcurrent supplied to the control circuit at the constant amplitude. Thiscondenser also serves the purpose of suppressing harmonics.

I shall describe two formsof apparatus embodying my invention, and shallthen point out the novel features thereof in claims.

In the accompanying drawing, Figs. 1 and 2' are diagrammatic viewsshowing two different a railway signal system but that this One use willserve to illustrate the many places where apparatus embodying myinvention will be useful.

Referring to Fig. 1, the, reference characters la and lb designate thetrack rails of a stretch of railway which are formed by the usualinsulated rail joints with a track section E-'-F, and which section maybe one section of a series of suc-' cessive sections of 'a signalsystem.The track section EF is provided with atrack circuit;

which includes a source of, coded alternating current connected acrossthe rails atone end of the section and a. code responsive track relayconnectedacross the rails at the other end of the section. According tomy invention, the source of coded alternating current for thetrack'circuit of the section EF includes an oscillating circuit 0C, acoupling transformer T, a coder CT and a suitable source ofunidirectional current, such as a battery BE.

The oscillating circuit 00 includes a winding'3 of a reactor R and acondenser 0 connected in series across primary winding 4 of transformerT. The winding 3 of reactor R is mounted on a magnetizable core 5 andthe oscillating circuit is proportioned for resonance at a preselectedfrequency, such second.

The coder CT may be any one of the several types well known to the art,and it is sufiicient for this application to point out that coder CT Iisprovided with a contact member 6 which is 1 be of any suitable coderatesuch as, for example,

as, for example, cyclesper 1-80-rcycles peraminute. The arrang'ement iipreferably such that contact member 6 when atlits first positionengagesaback contact' B and when raised to its second positiondisengag'es'this' contact 8, contact membenfi being at its two positions:for substantially one-half "of each cycle. As an "aid to theunderstanding of my invention, I shall refer to-the period in whichcontact member 6 engages contact 8 "as the-on period of the code-cycle,and the periodv in which contactmember-fi is raisedout of engagementwithcontact 8 as the off code period of the'cycle;

Battery BE is connected-across Winding 3 of reactor Rover contact -B--'8of coder 'C'I'. It follows that d-uring'each -on code'period contact6-45 is closed and direct curre'ntfiows from batteryBE "through reactor'winding? and magnetic energy isstored in'core 5 of the reactor, but thatwhen contact6--8 is opened each'o'ff' code periodthe'direct currentinwinding 3 isin't'errupted and the magnetic energy stored: in :c'ore 5'decays to 'create an electromotive 'forcein Winding 3. Suchelectromotive force "in turn 'sets up oscillations in the oscillatingcircuit '00, the

frequency oftheoscillation's being the natural frequency of the circuit"and which I have stated hereinbefore may be of theorderof 100 cyclesper second. These oscillations are damped, the oscillations having aninitial amplitude predetermined by the proper proportioning ofthe partsand dying away to zeroina period also predetermined by-the proportion oftheparts. Preferably, the damped wave of oscillation would "die out orat least fall to a -relatively-low value during the off code period ofcoder CT'so that theebove 'described'operation of creating a wave ofoscillations in the oscillating circuit is repeated'each cyclicoperation of coder CT.

A secondary Winding 9 of transformer T is connected across the rails laand lb of section Ee -F over wires 10 and H, a filter Fl comprising acondenser 12 and an inductor I3 being preferably interposed in wire -II. This filter Fl is tuned to .pass alternating current of the frequencycorresponding to that of the oscillations created in theoscillatingcircuit 0C, andin the case here assumed for illustration, itis tuned to pass alternating current of 100 cycles per second. It isclear that the oscillations flowing in primary winding 4 induce anelectromotive force of a corresponding frequency in secondary winding 9and alternating current of the same frequency is supplied to the trackcircuit through the filter Fl. Transformer '1 is characterized by asaturable core 14. That is, the parts are so proportioned that core i4is saturated by. current flowing in the primary winding 4 when suchcurrent is of an amplitude somewhat less than the'initia'l amplitude ofthe osc'illations created in the oscillating circuit 0G by the action ofcoder CT. "Consequently during the first part of the damped wave of theoscillations; the core 14 of transformer T is saturated and anelectromotive force of substantially constant amplitude isinduced insecondary winding 9 of the transformer during such first part of thedamped wave. After the oscillations have become too weak to supply therequired power the increase in the coupling when the transformer becomesi-msatnrated maintains the requisite power by increasing the rate ofwithdrawal of the remainoscillat'ory energy. Furthermore, by thusreducing the power drawn from the oscillating circuit during the earlypart of each wave of theoscillaticns, the rate at which the oscillationsforce Einduced in' tse'co'ndary "winding'iii to -be re- :Frcm thef'oregoin'gdescription, i-tis'to' be se'n thatLby i properl'prcportio'ning of 5 parts "the scode impulses of "alternating currentsupplied to I'the track circuit from secondary winding Strandde made tohave r-asubstariktially uniform-amplitude of a predeterminedvvalue and"to have "a nurs tion w i'chus substantially 'equalt'o th'e'oficOdeperiodof "coder CT. Since-a cod'e impulse of alternating "current issupplied ito the itrack cir cuit each off-'periodofcoder CT; thetracKci'r- "cuit current is "of: the same code rate as the operationcycle of coder CT-J Furthermore, such coded "alternating current willbes'ub'stantially free from harm'onics due to-filter El In Fig. '3thereiis illustrated the damped wave of the oscillations created in theoscillatin'g circuit 00 due tothe decaying o'fthe magnetic energy in thecore of reactor "R, arid Fi'g; 4 there is illu'strated the substantiallyconstant amplitudeelectromotive 'iorce induced in sec ondary winding9"in responseto the damped wave of t'he'cscillating circuit.

In-practicing the invention, the codingof-the direct current 'fsupplied"to"*reactor R woiild 'be governed by "traffic" conditions but suchco'ntrol is not 's'hcwm in the drawing isirrc'e it iform's' no art or'my invention andwbulu be in' accordanc e withstandard ractice. I

Anode responsive "relay T-R is connected across the rails of section E-F adjac'ellt the end F o'f thesec'tion by r'neans of the "usualtransformeri-ectifier combination comprisin a transformer TI and a full'w'ave'rec'tifier I5. Hes-0e track relay TB is supplied with anenergiiifg impnlse of'current curing earn code impulse or alternatcurrent supplied to the 'track ci'rbilit arid' the relay is operated ata rate corresponding to the rate of operational coder CT. Relay-TR wouldbe'l'is'ed to govern 'a signali'n'g circuit ror controlll'ng anysuitaiblsignalin devideassocia'teii with section When a train occupies sectionE-F,' track relay TR would heslili'ntedin the usual manner to effect acorrespondihg control of the associated signalin'g circuit. -'Alsb thecoded alternating current supplied to the track-circuit of 59651011 -EFWilt-11d be e'fibtlve to Glifilllbl ab signals by means "of any one ofthe several well known forms of "cab signal apparatus responsive tocoded alternating current. 7

Referring to Fig. 2, thetrack rails in anew of a stretch of railway areforined with 'a track section ii- F provided iliiith a track circuit thesame as in Fig. l. In Fig; 2, the means for sunplying codedalternatingcurrent to the 'tra'ck cir cult consists of an osc.atingic'ircuit O'Cl", a' coiipling transformer T2 a condenser "C2,coder CT and battery The oscillating circuitO'Cl or Fig. 2 incllldes-Windihg 3 6i reactor R], condenser C and a primary winding lli oftransformer T2; and is proportioned for a preselecteilnatural frequencythe s'aimeas the oscillating circuit-of Fig. 1. That is, oscillatingcircuit 0C! is resonant at apre'seie'cted frequency such as cyclespersecond. Consequently; a wave of oscillations isCreatedinoSCfllatlngcircuit OCI during each off code period of coder CTdue to the'decay of the magnetic energy stored in core 5 of reactorRdurin'ghthe- 'on code period of the coder: the

operation' being substantially the'Same-as described in detail inconnection with the oscillating circuit of Fig.1. 1

Transformer T2 is provided with a three-legged core structure, primarywinding 1 6 being mounted on one outside leg l7, and a secondary windingl8 being mounted on the other outside leg I9. The center leg 20 of thetransformer is of greater cross section than either outside leg I 1 orl9 and is preferably, provided with an air gap 2|. This core structureof'transformer T2 is proportioned forsaturation-of the outside leg l9 onwhich the secondary winding I8 is mounted when oscillations of, anamplitude somewhat less than the initial amplitude of the oscillationscreated in the oscillating circuit OCI due to the action of coder CTflow through primary Winding IE, but thatthe center leg 20 is notsaturated under such conditions. The other outside leg I! on whichprimary winding I6 is mounted would also be saturated under suchconditions. It is'clear that thecenter leg 20 forms a, magnetic shuntfor the outside legs I! and I9. Secondary winding [8 of transformer T2is connected'to the track circuit of section E-F overpwires-22 and 23..It follows that the oscillations caused to flow in primary winding 16induce an electromotive force of a corresponding frequency in secondarywinding l8 and alternating current of the same frequency is supplied tothe associated track circuit. Due to the saturable characteristics ofthe core structure of transformer T2, the amplitude of the electromotiveforce and in turn of the alternatin current supplied to the trackcircuit "will be substantially constant notwithstanding the oscillationsflowing in the primary winding are of a damped wave. Also each such codeimpulse of alternating current supplied to the track circuit may be madeto persist for substantially the full off code period Of coder CT. Anadvantage of transformer T2 is that when the outside legs of the corestructure are saturated and an increase in primary winding currentcauses an increase in the magnetic flux, most of the increment of theflux flows through the unsaturated leg 20 and hence the electromotiveforce induced in the secondary winding I8 is more constant in amplitude.Another advantage of transformer T2 is that the leakage reactance due tothe magnetic shunt, which is roughly equivalent to an inductance inseries with the secondary winding, tends to filter the output and reducethe harmonics of the current supplied to the-track circuit. 7

Condenser C2 is connected across the primary winding l B of transformerT2 and serves the purpose of tuning out the high inductance of primarywindin I6 at low amplitude of the oscillations and thus preventsfrequency variations of the oscillations due to variations of theoscillating circuit. Also, condenser C2 serves the purpose of maintaininthe critical required primary winding current after the oscillations inthe oscillating circuit have died down to a low'value. Furthermore,condenser C2 serves the purpose of smoothing out harmonics.

A track relay TR is connected across the rails of section E-F of Fig. 2in the same manner as in Fig. 1, and hence it is-operated by the codedalternating current supplied to the track circuit when the section isunoccupied and is shunted to control the associated signaling circuitwhen the section is occupied. Also the coded alternating current of thetrack circuit of Fig. 2' is effective to control cab signal apparatus inthe same man:

-ner aspointed out in connection with the apparatus of Fig. 1. g

It is to be seen, therefore, that I have provided novel and improvedmeans for supplying coded alternating current of a preselected frequencyand of substantially uniform amplitude to a control circuit from powerderived from a Source of unidirectional current. Also the duration ofeach such code impulse of alternating current is maintained atsubstantially its constant amplitude for the full on code period of thecurrent.

Although I have herein shown and described only two forms of apparatusfor generating coded alternating current embodying my invention, it isunderstood that variou changes and modifications may be made thereinwithin the scope of the appended claims without departing from thespirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination, a control circuit, a transformer having a saturablemagnetic core, a reactor having a winding mounted on a magnetizablecore, a capacitance; an oscillating circuit including in series saidreactor winding, said capacitance and a primary winding of saidtransformer and proportioned for resonance at a preselected frequency; asource of undirectional current, a coder having a cyclic operationconsisting of an on and an off period, means con. trolled by said coderto connectsaid current source across said reactor winding each said oncode period to store magnetic energy in the reactor core and todisconnect the current source from the reactor winding each said oifcode period to cause a damped wave of oscillations of said preselectedfrequency to flow in said oscillating circuit each said off code perioddue to the decay of the stored magnetic energy, and a secondary windingof said transformer connected to said control circuit to supply thatcircuit with a code impulse of alternating current in response to eachsuch wave of oscillations and which current is of substantially constantamplitude due to the saturable characteristic of said transformer core.

2. In combination, a control circuit, a transformer provided with amagnetic core saturable when oscillations of a preselected frequency andof a given amplitude flow in a primary winding of the transformer, a,reactor having a winding mounted on a magnetizable core, a capacitance;an oscillating circuit including in series said primary winding, saidreactor winding and said capacitance and proportionedfor resonance atsaid preselected frequency, a source of unidirectional current, a coderhaving a cyclic operation consisting of an on and an off period, meansincluding a contact of said coder to connect said current source acrosssaid reactor winding each said on period to store magnetic energy in thereactor core and to disconnect the current source from the. reactorwinding each said off period to cause oscillations of a damped wave formto flow in said oscillating circuit unimpeded by any energy stored insaid capacitance each said off code period and the first cycle of whichwave is greater than said given amplitude, and a secondary winding ofsaid transformer connected to said control circuit to supply to thatcircuit alternating current each coder off period in response to saidoscillations and which alternating current is of substantially uniformamplie tude because of the saturable characteristic of said transformercore.

3. In combination, a control circuit, a transformer provided with asaturable magnetic core and having a secondary winding connected acrosssaid control circuit, a reactor provided with a winding mounted on amagnetic core, a condenser, said reactor winding and said condenserconnected in series to a primary winding of said transformer to form anoscillating circuit proportioned for resonance at a preselectedfrequency, a source of unidirectional current, a coder having a contactmember alternately operated to a first and a second position at apredetermined code rate, and said source of unidirectional currentconnected across said reactor winding over the first position of saidcontact member to cause magnetic energy to alternately build up anddecay in the reactor core and the decay of which energy is effectiveeach time to cause damped oscillations to flow in the oscillatingcircuit and alternating current of said preselected frequency and ofsubstantially constant amplitude to be supplied to said control circuitdue to said oscillating circuit and the saturable magneticcharacteristic of said transformer.

4. In combination, a control circuit, a transformer having athree-legged magnetic core structure with a primary winding mounted on afirst one of said legs and a secondary winding mounted on a second oneof said legs, said secondary winding connected to said control circuit,a reactor having a winding mounted on a magnetizable core, a condenser;said reactor winding, said condenser and said primary Winding connectedin series to form an oscillating circuit tuned to resonance at apreselected frequency; a source of unidirectional current, a coderhaving a contact member which is operated at a predetermined code rate,said source of unidirectional current connected across said reactorWinding over said contact member to periodically create a wave of dampedoscillations in said oscillating circuit due to the periodic storing ofmagnetic flux in the core of said reactor and the periodic decay of suchflux, said first and second transformer core legs proportioned forsaturation by such wave of oscillations to cause a code impulse ofalternating current of said preselected frequency and of substantiallyuniform amplitude to be supplied to the control circuit, and the thirdtransformer core leg proportioned for non-saturation by such wave ofoscillations and provided with an air gap to form a magnetic shunt whichserves to filter the alternating current output and reduce theharmonies.

5. In combination, a control circuit, a transformer, having athree-legged magnetic core structure with a primary winding mounted onone outside leg and a secondary winding mounted on the other outsideleg, said secondary Winding connected to said control circuit, a reactorhaving a winding mounted on a magnetizable core, a condenser; saidreactor winding, said condenser and said primary winding connected inseries to form an oscillating circuit tuned to resonance at apreselected frequency; a source or unidirectional current, a coderhaving a cyclic code operation which consists of on and off periods,means controlled by said coder to connect said source of unidirectionalcurrent across said reactor winding each such on code period to create awave of damped oscillations in the oscillating circuit each off codeperiod due to the periodic storing of magnetic flux in the core of saidreactor and the periodic decay of such flux, said one outsidetransformer core leg proportioned for saturation by such wave ofoscillations flowing in the primary winding to cause a code impulse ofalternating current of said preselected frequency and of substantiallyconstant amplitude to be supplied to the control circuit, the centertransformer core leg proportioned for non-saturation by such wave ofoscillations and provided with an air gap to form a magnetic shunt; andanother condenser connected across said primary winding to preventfrequency variations in the oscillating circuit, to aid in maintainingthe duration of the code impulse of alternating current and to suppressharmonics.

AUSTIN M. CRAVATH.

